Extended x-ray absorption fine-structure study of Kr-Grafoil submonolayers

Abstract

Extended x-ray absorption fine-structure measurements were made at the Kr $K$ edge for 0.10, 0.20, and 0.35 monolayer coverages of krypton adsorbed on Grafoil, an exfoliated form of graphite. The Grafoil is measured to have 0.67±0.05 of its surface area oriented with rms angular variation of 15°. The adsorption site was explicitly determined to be the center of the carbon hexagons, and significant changes in this site were observed for the 0.10 monolayer coverage which were attributed to surface defects. The average Kr-C distance is determined to be 3.6±0.1 Å (corresponding to a mean distance of 3.35±0.1 Å between Kr and the graphite plane), and its mean-square deviation is found to increase by 0.051±0.009 ${\mathrm{\AA }}^2$ as the temperature changes from 10 to 100 K for the 0.35 monolayer. Krypton coordination is observed only for the 0.35 monolayer coverage; in this case the number of Kr nearest neighbors was found to be 1.8±0.5 and the distance to have the ($\sqrt{3}\times \sqrt{3}$) 30° value of 4.26±0.05 Å. The Kr-Kr mean-square deviation and the third moment of the radial distribution of 10 K were determined to be 0.0166±0.004 ${\mathrm{\AA }}^2$ and 0.0010±0.0003 ${\mathrm{\AA }}^3$, respectively. The small Kr-Kr coordination is inconsistent with the generally accepted phase diagram.